Abstract
Molecular mechanisms of Na+, Cl−, and Ca2+ regulation in ionocytes of fish have been well investigated. However, the regulatory mechanism of K+ in fishes has been largely unknown. In this study, we investigated the mechanism of K+ regulation in medaka larvae acclimated to fresh water. Using a scanning ion-selective electrode technique (SIET) to measure the K+ fluxes at skin cells, significant K+ effluxes were found at ionocytes; in contrast, significant K+ influxes were found at the boundaries between keratinocytes. High K+ water (HK) acclimation induced the K+ effluxes at ionocytes and suppressed the K+ influxes at keratinocytes. The K+ effluxes of ionocytes were suppressed by VU591, bumetanide and ouabain. The K+ influxes of keratinocytes were suppressed by TAP. In situ hybridization analysis showed that mRNA of ROMKa was expressed by ionocytes in the skin and gills of medaka larvae. Quantitative PCR showed that mRNA levels of ROMKa and NKCC1a in gills of adult medaka were upregulated after HK acclimation. This study suggests that medaka obtain K+ through a paracellular pathway between keratinocytes and extrude K+ through ionocytes; apical ROMKa and basolateral NKCC1a are involved in the K+ secretion by ionocytes.
Highlights
Fishes encounter harsh ionic and osmotic gradients derived from their aquatic environments, and the mechanisms for maintaining internal homeostasis are more challenging for fishes compared to terrestrial vertebrates[3]
Whole-body ion contents of larvae acclimated to K+-free water (KF), normal water (NW) and high-K+ water (HK)
Using the selective electrode technique (SIET), K+ fluxes at specific cells were measured on the surface of skin in medaka larvae acclimated to NW (Fig. 2)
Summary
Fishes encounter harsh ionic and osmotic gradients derived from their aquatic environments, and the mechanisms for maintaining internal homeostasis are more challenging for fishes compared to terrestrial vertebrates[3]. The molecular mechanisms of Na+, Cl−, and Ca2+ uptake by specific types of ionocytes were revealed in several fish species including zebrafish, tilapia, trout, and medaka[4,5,6,7]. NKCC1 was found in the basolateral membrane of ionocytes in FW-acclimated fishes including tilapia[19], rainbow trout[20], milkfish[21], and medaka[7], expression levels were not as high as those in SW-acclimated fishes. NKCC1 in basolateral membranes of ionocytes was hypothesized to be involved in K+ secretion by medaka in FW. Previous studies suggested that ROMKa mediates K+ secretion by ionocytes of tilapia[9,11] and zebrafish[10], convincing evidence of K+ flux at ionocytes is still lacking. Japanese medaka (Oryzias latipes) larvae were used as an animal model because previous studies showed it to be an ideal model for functional studies of ionocytes[22,26,28]
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